1. Field of the Invention
The invention relates to a capillary flow cell that is used for light absorption measurements and, more particularly, to a bent capillary flow cell having bulbous ends to facilitate entry of light into the flow cell.
2. Description of the Background Art
Light absorption detectors for high performance liquid chromatography (HPLC) and capillary electrophoresis (CE) generally comprise four components: a light source, a means for selecting a narrow increment of wavelengths, a flow cell through which the sample to be analyzed and the light are passed, and a light sensor that measures the amount of light transmitted through the flow cell. When a light absorbing fluid sample passes through the flow cell, the amount of transmitted light decreases in accordance with Beer's Law; ##EQU1##
where I is the transmitted light power, I.sub.0 is the light power incident on the flow cell, a is absorptivity of the sample, b is the pathlength of the flow cell, and c is the sample concentration. The units of a, b, and c are chosen so that the product a b c is dimensionless. This product is defined as absorbance (A) and is the customary output of light absorption detectors. The absorbance output (A) of the detector increases in direct proportion to pathlength for a given change in sample concentration. But increasing the cell pathlength, without at the same time decreasing the cross sectional area of the cell, increases the volume of the cell and dispersion of the sample as the sample flows through the cell. This peak spreading is detrimental to the separating efficiency of the chromatographic or electrophoritic system in which the detector is used. For this reason, cells with capillary dimensions are sometimes used in HPLC and CE.
Chervet in U.S. Pat. No. 5,057,216 and Moring in U.S. Pat. No. 5,274,227 describe bent capillary flow cells where light absorption is measured in an elongated straight section of capillary tubing between two bends in the tubing. The sample containing liquid enters and leaves the flow cell via the two bends. Light is introduced into and extracted from the cell through the outside walls of the bends in the tubing. These bent capillary flow cells can offer a long liquid pathlength for the light, small cell volume, and a cleanly swept flow path through the flow cell. Chervet in U.S. Pat. No. 5,057,216 describes a method of manufacture and mounting a bent capillary flow cell, while Moring in U.S. Pat. No. 5,274,227 describes in detail the high degree of collimation and narrow width of the illuminating light beam required for efficient coupling of light into the liquid containing bore of the flow cell through the outside of a bend in the capillary tube. Moring also describes the angle and lateral offsets of the incident light beam with respect to the centerline of the elongated straight section on the bent capillary flow cell and maximum bend radius in order for the incident light beam to completely illuminate the bore of the elongated straight section of the cell.
The light throughput of a small aperture in an optical system T is approximately defined as EQU T=n.sup.2 A.OMEGA. (2)
where n is the refractive index in the space where the aperture is located, A is the area of the aperture, and .OMEGA. is the solid angle included in the cone of light defined by limiting rays that pass through the aperture. It is desirable that a flow cell has high light throughput so that the light sensor and its amplifier electronics give a signal with a high signal-to-noise ratio (SNR). The narrow range of acceptance angles and the narrow lateral width of the illuminating light beam in the detector flow cells described by Chervet and Moring necessarily limit the light throughput of their flow cells. Furthermore, it is difficult to control and minimize the component of light that is lightpiped within the wall of the flow cell. This component of light travels almost entirely in the cell wall and has little exposure to the sample in the cell. As such, this stray light contributes to the noise of the detector without much contribution to signal. Additionally, this stray light can also severely limit the linearity of the detector response for a change in sample concentration. To illustrate this last point, assume that half of the incident light travels along the cell by light piping within the wall where the light absorbance of the sample is zero, and half of the incident light travels through and parallel to the cell bore where the absorbance is A.sub.0. The absorbance measured by the detector according to Beer's Law (1) is ##EQU2##
The slope of the detector response curve is the derivative of A with respect to A.sub.0. ##EQU3##
The slope of the response curve is 1/2 for A.sub.0 equal to 0 and is 1/11 for A.sub.0 equal to 1. Clearly the detector response is highly nonlinear with change in A.sub.0 which is itself proportional to change in sample concentration.
Therefore, there is a need in the art for an improved capillary flow cell.